The present invention generally relates to substrates having semiconductor-on-insulator structures and production methods thereof, and more particularly to a substrate having a semiconductor-on-insulator structure with gettering sites and a production method thereof.
A substrate having a silicon-on-insulator (SOI) structure (hereinafter simply referred to as an SOI substrate) is effectively used as a semiconductor crystalline substrate for realizing a high-speed operation of a semiconductor device.
Generally, when producing a semiconductor device having a semiconductor crystalline substrate, it is necessary to carry out a gettering to eliminate crystal defects and impurities which are generated during a production process of the semiconductor device from an active layer in which semiconductor elements are formed. However, in the case of a semiconductor device having the SOI substrate, it is difficult to realize the gettering function due to its structure. This difficulty in realizing the gettering function in the SOI substrate is an obstacle to obtaining a high performance in the semiconductor device having the SOI substrate.
As methods of forming the SOI substrate, there basically are two methods. According to a first method, an SOI substrate is first formed, and a gettering site is formed thereafter. In FIG. 1A, the SOI substrate comprises a substrate 1, a silicon dioxide (SiO.sub.2) layer 2 which is formed on the substrate 1, and a silicon (Si) layer which is formed on the SiO.sub.2 layer 2. This SOI substrate is subjected to a thermal process so as to form a gettering site 4 in the Si layer 3 as shown in FIG. 1B. However, the coefficients of thermal expansion of SiO.sub.2 and Si are different, and it is undesirable to subject the SOI substrate to the thermal process which is carried out at a relatively high temperature in the order of 1400.degree. C. for a relatively long time. In addition, because the thickness of the Si layer 3 is in the order of 5 microns and small, the gettering site 4 is small and unsatisfactory for carrying out the intended elimination of the crystal defects and impurities from a denuded zone 3a of the layer 3.
According to a second method, two stacked structures respectively made up of a single crystal Si substrate and an SiO.sub.2 layer are prepared and the SiO.sub.2 layers of the two stacked structures are bonded to form a so-called bonded substrate. This method is a more promising method than the first method due to recent improvements in bonding techniques. But it is impossible to realize the gettering function in the SOI substrate which is produced by this second method.
As methods of forming gettering sites in the substrate, there are the extrinsic gettering and the intrinsic gettering. The extrinsic gettering forms the gettering sites by applying an external distortion to a back surface of the Si semiconductor substrate. On the other hand, the intrinsic gettering uses the Si semiconductor substrate which is formed by the Czochralski method, for example. Normally, the Si semiconductor substrate formed by the Czochralski method includes oxygen (O.sub.2) in the order of 1.times.10.sup.18 cm.sup.-3, and thus, the extrinsic gettering forms the gettering sites by subjecting the Si semiconductor substrate to a thermal process to precipitate SiO.sub.2.
In the case of the SOI substrate formed by the second method, the SiO.sub.2 layer exists at the central portion and it is impossible to employ the extrinsic gettering to form the gettering sites.
In addition, it is also impossible to employ the intrinsic gettering to form the gettering site in the SOI substrate which is formed by the second method for the following reasons. That is, in the SOI substrate, the thickness of a single crystal semiconductor layer in which semiconductor elements are formed needs to be in the order of 5 microns, and it is virtually impossible to form both an active layer for forming the semiconductor elements and the gettering sites in this extremely thin single crystal semiconductor layer.
Normally, when realizing the gettering function in the Si semiconductor substrate which is formed by the Czochralski method, the Si semiconductor substrate is subjected to a thermal process at a temperature of over 1000.degree. C. so as to diffuse the O.sub.2 in vicinities of the front and back surfaces of the Si semiconductor substrate and form a denuded zone. Then, a thermal process is carried out at a temperature in a range of 400.degree. C. to 600.degree. C. to form gettering sites. Next, a thermal process is carried out at a temperature of over 1000.degree. C. to grow the gettering sites.
The semiconductor elements are formed in the denuded zone, but the thickness of the denuded zone is in the order of 20 microns. Hence, even when a stacked structure 10 made up of a base substrate 11 and a SiO.sub.2 layer 12 is bonded to another stacked structure 15 made up of a Si active substrate 16 and SiO.sub.2 layers 17a and 17b and having gettering sites 18 formed by the intrinsic gettering as shown in FIG. 2A, the SiO.sub.2 layer 17a and the Si substrate 16 must be polished so as to leave an active layer which has a thickness in the order of 5 microns. As shown in FIG. 2B, the gettering site 18 will be removed by the polishing as indicated by a phantom line and only the active layer remains according to this method. Therefore, in the case of the SOI substrate formed by the second method, it is also impossible to employ the intrinsic gettering to form the gettering site.
Therefore, in the case of the SOI substrate formed by the second method, it is impossible to form a gettering site.
Because it is conventionally impossible to realize the gettering function in the SOI substrate, there are problem in that it is impossible to eliminate by the gettering the crystal defects and impurities generated during the production process of the semiconductor device which has the SOI substrate, and there is no means to prevent deterioration in the characteristics of the semiconductor elements which are formed in the active layer.